1. Field of the Invention
The present invention relates to a method and apparatus for calibrating high-resolution multi-element sensors which are mounted in artificial satellites or the like for earth observation, or in facsimile machines or the like for detecting optical images, etc.
2. Description of the Prior Art
Multi-element sensing apparatuses mounted in facsimile machines, artificial satellites, etc. perform electric calibration at regular time intervals to keep track of the operational state of electric circuits throughout the equipment while detecting image signals. Multi-element sensing apparatuses of the prior art are designed to receive image signals when the subject being photographed is well lit, and to perform electric calibration while the subject being photographed is poorly lit.
Particularly, in the multi-element sensing apparatuses used for the artificial satellites, electric calibration signals having a predetermined level are applied to a CCD part in the sensing apparatuses in an observation-off time and the resultant calibration output signals are transmitted to an earth station as the same as observation output signals (image signals). In the earth station, the observation output signals are calibrated using the resultant calibration output signals. FIG. 7 schematically shows such image signals in time sequence, including outputted image signals 16, outputted electric calibration signals 18, and outputted zero (or offset)-level signals 17. As illustrated therein, all the image signals 16, the electric calibration signals 18 and the zero (or offset)-level signals 17 are outputted to the same-numbered pixels.
With conventional multi-element sensing apparatuses of this type, electric calibration is performed only when the subject being photographed is placed under low light conditions, since electric calibration for the subject being photographed in lit places results in overlapping of image signals on electric calibration signals. Therefore, some limits are imposed on the timing of the performance of electric calibration, resulting in the drawback of preventing calibration measurements whenever necessary. Further, according to the conventional apparatuses, it is impossible to realize accurate and continuous calibration when a condition such as temperature is changed during an observation time.
To improve such conventional apparatus, a technique applying a particular register having additional stages for calibration sequences is disclosed in U.S. Pat. No. 5,317,423. However, this technique requires a special sensing device having such additional stages which is disadvantageous with respect to circuit configuration and cost.